Electric impedance tomography (EIT) is a process, which is known per se, in which the electric impedance between the feed point and the test point can be calculated by feeding an alternating electric current of, e.g., 5 mAeff at 50 kHz into any electrically conductive body, here preferably into the human body, and measuring the surface potentials resulting therefrom at different points of the body. A two-dimensional tomogram of the electric impedance distribution in the body being considered can be determined on the basis of suitable mathematical reconstruction algorithms by gradually rotating the current feed sites around the body while measuring at the same time the surface potentials along a section plane.
Such a tomograph of the impedance distribution of the human body is of interest in medicine because the electric impedance changes both with the air content and the extracellular fluid content in the tissue. It is thus possible to visualize and monitor with this process especially the ventilation, i.e., the ventilation of the lungs, as well as the changes in the end-expiratory lung volume in a regionally resolved manner.
It is known that ventilated lung areas as well as the changes therein over time can be represented by means of EIT.
It was reported that the recovery from abdominal surgical procedures normally takes place rapidly and without complications (Squadrone et al., Continuous Positive Airway Pressure for Treatment of Postoperative Hypoxemia in JAMA, Feb. 2, 2005, Vol. 293, No. 5, pp. 589-595), but 30% to 50% of the patients nevertheless develop postoperative hypoxemia as a consequence of the development of atelectasis, this happening even in patients in whom no postoperative complications developed. Even though the administration of oxygen and breathing training can be employed highly efficiently in most cases, respiratory failure may develop during the early postoperative phase, which calls for endotracheal intubation and mechanical ventilation in 8% to 10% of this patient group, which may in turn lead to complications in the hospital. It is therefore important to recognize a change in the lung status as early as possible and to set the respirator (ventilator) such that exacerbation of the patient's condition is counteracted in time.
This is especially true in connection with surgical procedures during which atelectases frequently develop.